Modification or treatment of a surface by applying glow plasma is a known technique in industries, such as photo film production industry, used in order to improve certain surface and material properties. For instance, in the production of photo film, a thermoplastic polymer film (triacetyl cellulose (TAC), polyethyleneterephthalate (PET), polyethylene-naphthalate (PEN) or similar) is prepared using a glow plasma in order to improve adhesion properties of the surface.
Plasma is considered generally as a suitable solution for material processing, because it generates a large flux of reactive species (radicals, ions), which can be directed to the process zone and manipulated to the desired shape by using an appropriate electric field distribution. Plasma treatment would have considerable advantage if it could be generated at atmospheric pressure. Advantages of using atmospheric pressure are a larger density of reactive species than in the low pressure case, and the advantage of avoiding vacuum technology.
Another desired feature of atmospheric pressure glow plasmas (APG) is the generation of these plasmas at low temperatures around 300-400 K. This will make the technology applicable to the treatment of thermoplastic polymer surfaces, as is common in photo film production methods.
Generating a plasma under the above circumstances is not a straightforward technique. At atmospheric pressure, the particle density is high and as a result the mean free path of reactive species is small. The processes of excitation and ionisation are restricted to a limited area, and the plasma is generated primary in a filamentary form.
Plasmas at atmospheric pressures are very unstable and will tend to go into a spark or an arc in short time after the breakdown. Any random local increase in a current density will tend to grow rather than to be damped and plasma will be constricted.
Generating a plasma requires the supply of sufficient energy to a gas such that the gas is ionised. Within the plasma, collisions and interactions between elements of the gas create chemically or physically active species, such as metastables, ions, electrons, and others. Recombinations and transitions of excited elements to their ground state also causes the emission of photons from the plasma.
For the plasma to be sustained, sufficient free electrons should be present in the plasma. One of the solutions for obtaining a homogenous atmospheric plasma is the generation of a background pre-ionization, implying that sufficient seed electrons must be present in the reactor before the plasma breakdown. These seed electrons can be created as described above, through interactions within the plasma itself or can be generated as a result of interactions between the species present in the plasma and the surface of the electrodes.
In general an APG plasma is created by applying an AC-voltage to a plurality of electrodes. A substrate to be treated, such as a thermoplastic polymer film, may be transported along the surface of one or more of these electrodes. The frequency of the AC-voltage may be varied in order to improve the properties of the plasma.
The applicability of a plasma for use in material processing may be evaluated by determining the following parameters:                the power coupled into the APG plasma calculated from the applied voltage and the plasma current;        the exposure non-uniformity parameter, based on the statistical distribution of the exposure over the surface;        the minimum time of exposure, wherein 99% of the surface is exposed to a surface plasma energy dose of at least 1 J/cm2 (statistical fluctuations of the exposure taken into account).        
The abovementioned exposure non-uniformity parameter and minimum time of exposure will be described in more detail below.
In a number of situations and applications, such as in material processing in photo film industry, it will be advantageous to have the plasma treatment time as short as possible. The plasma exposure time t is given by:
                    t        =                  L          v                                    (        1        )            wherein L is the length of the plasma reactor and v the line speed.
A short time of exposure allows the use of high line speeds and of “medium” size of APG reactors (L=10-20 cm). The energy dose required for reaching the maximum surface energy at the surface of the material to be treated may, as will be appreciated, depend on a number of factors, amongst which are the properties of the surface and the material. It is accepted that an atmospheric pressure glow plasma treatment with an energy dose of 1 J/cm2 will be sufficient to reach the maximum surface energy. However in order to get an acceptable adhesion between a certain type of polymer such as Polyethylene (PE) and gelatin an energy dose of between 50 and 100 mJ/cm2 may already be sufficient.
The minimum time of exposure required for treating a material is given by:
                              t          minim                =                              SE            minim                    P                                    (        2        )            
wherein P is the power consumed by plasma and S the active electrode surface, i.e. the part of electrode surface which is covered by plasma.
Since the plasma exposure is a stochastic process, whether or not a local part of the surface is exposed to the plasma is stochastic as well. This is represented by a stochastic parameter with an average value of P/S and with a certain dispersion from this average value. The minimum time of exposure is therefore determined by the requirement that with a certitude of 99% all elements of the surface are exposed to an energy dose of at least 1 J/cm2. Depending on the plasma uniformity, the energy dose exposed to the surface may be considerable higher in some local sites at the surface.
Taking in account the statistical fluctuations of the exposure the minimum time of exposure is given by:
                              t          minim                =                              SE            minim                                P            ⁡                          (                              1                -                                  3                  ⁢                                                                          ⁢                                                            σ                      ⁢                                                                                          ⁢                      D                                                              D                      _                                                                                  )                                                          (        3        )            wherein D is the energy dose received locally at the surface and σD the statistical dispersion (the mean standard deviation from the average value). The statistical dispersion of the exposure is calculated later in this document.
Another quality criterium of the treatment process is the uniformity of the plasma exposure. In order to establish a high level of uniformity it is required that fluctuations of the exposure are present only at microscopic scale (not detectable by the human eye: of the order of 100 μm or smaller). So with a certitude of 99%, any element of the surface of the material having a size of 10−4 cm2, must be exposed to an energy dose of at least 1 J/cm2.
It is important to note that for streamer discharges, like a corona or a silent discharge, it is very difficult to meet the criteria mentioned above. The cause of it is believed to be the repulsive forces of streamer space charge and the drop of the electric field near a streamer plasma. As a result, this type of discharge can not cover all the material to be treated and gaps of a few millimeters exist between the streamer discharges.
For a single pulse the dispersion (average relative variation) of the exposure over a surface unit of 10−4 cm2 is given by:
                                          σ            ⁢                                                  ⁢                          D              sp                                                          D              _                        sp                          =                                            σ              ⁢                                                          ⁢                              (                                  E                  pulse                                )                                                    E              pulse                                ·                                    S                              10                                  -                  4                                                                                        (        4        )            wherein Dsp is the energy dose per unit of surface per pulse, S is the electrode surface in cm2 and Epulse is the energy delivered to the plasma by a single pulse. For obtaining equation (4) it was assumed that the value of the current density on each element of surface is statistically independent.
Assuming a Poisson statistical distribution of the exposure, the relative fluctuations of exposure for the exposure to N pulses will be:
                                                        σ              ⁢                                                          ⁢              D                                      D              _                                =                                                                      σ                  ⁢                                                                          ⁢                                      (                                          E                      pulse                                        )                                                                    E                  pulse                                            ·                                                S                                      10                                          -                      4                                                                                            *                          1                              N                                                    ⁢                                  ⁢                                            σ              ⁢                                                          ⁢              D                                      D              _                                =                                    1                              ft                                      ⁢                                                            σ                  ⁢                                                                          ⁢                                      (                                          E                      pulse                                        )                                                                    E                  pulse                                            ·                                                S                                      10                                          -                      4                                                                                                                              (        5        )            
herein f is the pulse frequency and N=ft is the number of plasma pulses received by an element of the surface, during movement of a material surface (for example a surface of a foil) through the APG reactor.
The value of the statistical fluctuation of the energy per pulse can be determined from the waveforms of the plasma current and the applied AC-voltage using 16 sample pulses. HenceσEpulse=σ(∫Iplasma(t)U(t)dt  (6)wherein Iplasma is the plasma current and the U is the AC-voltage applied to the electrodes of the APG reactor.
Taking in account statistical fluctuations of exposure and equation (2), the minimum time of exposure is determined as follows:
                                                                        P                S                            ⁢                                                          ⁢                                                t                  minim                                ⁡                                  (                                      1                    -                                          3                      ⁢                                                                                          ⁢                                                                        σ                          ⁢                                                                                                          ⁢                          D                                                                          D                          _                                                                                                      )                                                      =                                          1                ⁢                                                                  ⁢                J                ⁢                                  /                                ⁢                                                      cm                    2                                    ⁢                                                                          ⁢                                                                          ⇓                                                                          ⁢                                                            P                      S                                        [                                                                  t                        minim                                            -                                              3                        ⁢                                                                                                            t                              minim                                                        f                                                                          ⁢                                                  (                                                                                                                    σ                                ⁢                                                                                                                                  ⁢                                                                  (                                                                      E                                    pulse                                                                    )                                                                                                                            E                                pulse                                                                                      ·                                                                                          S                                                                  10                                                                      -                                    4                                                                                                                                                                                )                                                                                      ]                                                              =                                                1                  ⁢                                                                          ⁢                  J                  ⁢                                      /                                    ⁢                                                            cm                      2                                        ⁢                                                                                  ⁢                                                                                  ⇓                                                                                  ⁢                                          t                      minim                                                                      =                                                      (                                          F                      +                                                                                                    F                            2                                                    +                                                                                    SE                              minim                                                        P                                                                                                                )                                    2                                                              ;                ⁢                                  ⁢                  F          =                                    3              2                        ⁢                                          1                f                                      ⁢                          (                                                                    σ                    ⁡                                          (                                              E                        pulse                                            )                                                                            E                    pulse                                                  ·                                                      S                                          10                                              -                        4                                                                                                        )                                                          (        7        )            
Equations (5) and (7) clearly reveal the importance of a high frequency in order to reduce the time of exposure and to increase the uniformity. The minimum time of exposure, defined by equation (7), is strongly dependent on the requirements for the treatment of PEN, PET and PE for improvement of coating adhesion, and further depends on the minimum exposure and the size of surface on which significant non-uniformities are admissible.
A parameter which reflects the plasma uniformity and which can be used to compare atmospheric pressure glow (APG) plasmas for different working conditions and applications, is the variation of exposure over a surface of 1 cm2 (after a single pulse exposure):
                              δ          ⁢                                          ⁢          D                =                                            σ              ⁢                                                          ⁢                              D                1                                                                    D                _                            1                                =                                                    σ                ⁢                                                                  ⁢                                  (                                      E                    pulse                                    )                                                            E                pulse                                      ·                                          S                1                                                                        (        8        )            In the following δD (measured as a percentage) will be named exposure non-uniformity parameter hereinafter, and will be used as a plasma quality parameter together with the minimum time of exposure, as defined by equation (7).
The exposure uniformity may be checked by a toner test, visualising the charge distribution of charge on the surface.
In this case the ratio d between the size of the toner build-up spots and the distance between the toner build-up spots is:
                    d        =                                            S              π                                ·                                    σ              ⁢                                                          ⁢                              (                                  E                  pulse                                )                                                    E              pulse                                                          (        9        )            
Several attempts have been made in order to generate a stable atmospheric glow plasma.
In U.S. Pat. No. 6,299,948 there is disclosed a method for generating a uniform plasma through a proper combination between the AC voltage and the frequency in a nitrogen gas. The frequency range used is 200 Hz to 35 kHz, with a preference frequency below 15 kHz, and an amplitude of the applied voltage in the range between 5 to 30 kV.
However, processing of materials in plasma at higher frequencies is beneficial due to the increased number of discharges over time. Although the charge generated per pulse and the energy per pulse are only slightly changing with the frequency, when the operating frequency is increased the number of pulses per unit of time increases. Therefore the average power (energy released per unit of time) increases and the average current (charge released per unit of time) increases almost proportionally with the frequency. As a consequence, the material processing is faster than at lower frequencies and, as will be appreciated, this presents a clear benefit for the industrial process.
U.S. Pat. No. 5,414,324 describes an apparatus and method for generating a uniform atmospheric pressure glow discharge plasma in a frequency range of 1 to 100 kHz. The frequency ranges used in this document are based on the model of ion trapping, i.e. where mobility of the ions is so low that they will be trapped in the plasma gap whereas the mobility of the electrons is sufficiently high and can reach the electrodes. A gas is present between the discharge electrodes, which gas at least comprises one of air, nitrous oxide or a noble gas such as argon, helium, neon, etc. The document however suggests that for higher frequencies the plasma becomes unstable and cannot be sustained anymore.
The application of helium as reaction gas at high frequency plasma generation has been performed more often. European patents EP 0 790 525, EP 0 821 273 and EP 0 699 954 disclose methods and arrangements for generating a glow plasma at high frequencies.
EP 0 790 525 discloses the generation of plasmas at 10 kHz in air and 40 kHz, 450 kHz and 13.56 MHz in He with small amounts of N2 and/or O2. The gas is purged through holes in the electrodes of the arrangement used. Measurements of the adhesion properties of plasma treated PET and PEN reveals that an operating frequency of 450 kHz is favourable to improve adhesion effectiveness. Both corona and plasma treated samples are compared in these documents showing that plasma provides superior adhesion properties.
An improved embodiment in EP 0 821 273 describes the adhesion properties of plasma treated PEN, wherein the plasma is generated in (combinations of) helium and nitrogen as compared to air for use as a reaction gas, at frequencies of 10 and 450 kHz. The arrangement used is a corona-like setup comprised of bare titanium parts positioned in parallel to a drum. The drum is covered with a silicone layer and a polymeric film is transported on the drum through the discharge space. The discharge space is filled with a gas essentially comprising helium and the polymeric film is exposed to an atmospheric glow discharge. Some experiments were performed with a reaction gas essentially comprising N2 at a frequency of 10 kHz.
In U.S. Pat. No. 5,585,147 there is disclosed a method for treating the surface of a glass fabric with an atmospheric pressure glow discharge plasma for cleaning the surface, improving the adhesion properties thereof and coating the surface with a organosilane compound. The glass fabric is treated with a plasma generated in a carrier gas which is essentially comprised of helium and/or argon, and which carrier gas is mixed with a reaction gas. In order to achieve the desired cleaning performance and adhesion properties, the gas mixture is preheated to temperatures between 100° C. and 600° C., before generating the plasma. These temperatures however make this plasma unsuitable for surface treatment of temperature sensitive substrates, such as thermoplastic polymer films.
The application of helium however in methods for generating APG plasmas at high frequency (>50 kHz) of the AC-voltage, is not favourable. Helium occurs in the atmosphere in a concentration of approximately 5 ppm (parts per million). Some natural gas deposits however have been found to contain significant amounts of helium and therefore most helium is obtained from these national gas deposits. Some of the deposits contain helium in a concentration above 0.3% by volume. Most of the helium in the world comes from natural gas deposits found in Texas, Oklahoma, Kansas and The Rocky Mountains.
Helium is extracted from natural gas streams using a low temperature liquefaction process. This low temperature method separates crude helium—a mixture of more than 50% helium with nitrogen and small amounts of other gasses—from the liquefied portion which consists predominantly of hydrocarbons. Several other techniques including pressure swing adsorption (PSA) and some cryogenic processes, are used to refine crude helium into a concentrated helium product. Helium is an expensive gas due to the fact that it is rare and difficult to extract. It is mainly for this reason that the use of helium is preferably avoided in applications described above.
Another drawback of the application of helium is that due to the specific weight of helium (0.17 g/l), helium is very volatile. For this reason helium gas will always try to escape through the smallest holes of the arrangement. More countermeasures such as appropriate sealing and a more complex system for the gas injection means are required in arrangements using helium as a reaction gas.
Lastly, helium with an atomic weight of 4 will release a relatively small amount of energy in a collision with the surface to be treated. The use of larger, more heavy particles will exchange larger amounts of energy and will assist in the physical treatment of the surface. Therefore, in applications such as surface activation and etching, helium will not be the most favourite choice.
Further to this, heating of the carrier gas in the discharge space, such as is done in U.S. Pat. No. 5,585,147, is not a preferred option for treating thermoplastic polymer films, such as is done in photo film production industry. The higher temperatures may damage the thermoplastic polymers, for instance due to melting thereof. On the other hand, it has been experienced that the temperature automatically rises to some extend due to energy dissipation.